The invention relates to a spindle motor and a hard disk drive.
A spindle motor for a hard disk drive generally comprises a rotating component, a rotor, having an annular permanent magnet and a stationary component, a stator, having a stator stack wound with the coils, wherein the rotor is equipped with an appropriate rotary bearing to support the rotor with respect to the stator.
Alongside rolling bearings which have been in use for a long period of time, hydrodynamic bearings are now finding increasing application. A hydrodynamic bearing is an improvement on a journal bearing and can include a bearing sleeve having a cylindrical inner bearing surface and a shaft having a cylindrical outer bearing surface, the shaft being set into the sleeve. The diameter of the shaft is only slightly smaller than the inside diameter of the sleeve, thus a radial bearing gap is created between the two bearing surfaces that is filled with a lubricant, preferably oil, forming a continuous capillary film.
Bearing oil can be prevented from escaping from the hydrodynamic bearing by providing one end face of the bearing sleeve with an airtight seal. At the opposite open end a tapered area can be formed between the shaft and the inner surface of the bearing. The tapered area accommodates expanded volume of the lubricant during operation and acts as a lubricant reservoir. This tapered area also takes on the function of sealing the bearing by means of a capillary tension phenomenon. Under the influence of capillary forces, the oil in the area between the shaft and the tapered opening of the bearing sleeve forms a stable, continuous liquid film. The described seal is often called a capillary seal.
The main factor limiting the lifespan of a spindle motor with a hydrodynamic bearing is decrease in quantity of lubricant over time because a part of the bearing oil transforms into a gaseous state as a result of vapor pressure. The vaporization rate is directly proportional to the temperature and inversely proportional to the pressure on the surfaces contacting the surrounding air.
With the loss of lubricant, the risk that the bearing surfaces come into contact with each other when the motor starts up and stops increases. This causes particles, which may be larger than the thickness of the bearing gap, to be rubbed off. This results in dry running and seizure of the bearing surfaces leading to blockage of the spindle motor.
In addition, the vaporized matter can end up being deposited, iner alia, on the data disks in the hard drive which could result in the “bonding” of the read/write head to the data carrier disks leading again to a total breakdown of the hard disk drive.
A capillary seal with a tapered area contributes to the vaporization process in that the bearing oil on the surfaces in contact with the air forms a meniscus and, with a relatively large surface at the beveled inside edge of the bearing sleeve, is exposed to the surrounding air.